It is known to supply electric energy to a winding of a reversible permanent magnet D.C. electric motor by means of a circuit arrangement that is often referred to as an H-bridge motor control circuit and includes electric conductors which respectively connect each of the ends of the winding with each of the negative and positive terminals of an electric current source. In this type of a motor control circuit, a control switch is interposed in each of the aforementioned electric conductors between each of the terminals and each of the winding ends. Then, it is possible to control the operation of the electric motor, that is, its energization for rotation in one or the other of its senses of rotation, a reversal in its sense of rotation, its dynamic braking, and its de-energization, by causing selected ones of the control switches to assume their closed positions in which they permit the passage of electric current therethrough while causing the remaining control switches to assume their open positions in which they prevent passage of electric current therethrough.
In operating an electric motor utilizing the aforementioned H-bridge motor control circuit, it is sometimes or even frequently necessary to accomplish a reversal in the sense of rotation of the electric motor from a relatively high or even full speed in one sense to a relatively high or even full speed in the opposite sense. Under these circumstances, magnetic flux fields existing in the electric motor at the beginning of the reversal induce an electric current spike in the motor winding, inasmuch as the electric motor acts as a generator during its slowdown. Simultaneously with the occurrence of this induced electric current surge, the gradient of the voltage differential supplied from the electric power source to the winding is reversed by switching the previously closed control switches into their open positions and the previously open control switches into their closed positions. This combination of electromagnetic phenomena results in a total magnitude of the electric current spike, when the called-for speed of rotation reversal is considerable, which is larger and often much larger than the electric current spike encountered during the start-up of the electric motor from standstill. As a matter of fact, when the reversal is to be from full speed of rotation in one sense to full speed of rotation in the opposite sense, the magnitude of the electric current spike is about twice that of the normal start-up current spike.
Obviously, this is very disadvantageous since this relatively high electric current spike may damage the electric motor circuitry which is usually designed or rated on the basis of the normal start-up current spike. Moreover, the amplitude of the current spike may be so large as to exceed the current rating of the motor, which leads to at least partial demagnetization of the permanent magnets used in such a motor. When this happens, a larger electric current than that required otherwise is needed for operating the motor. As a consequence, the winding, transistors and motor efficiency all suffer. To avoid these problems, resort has been made to overdesign of the electric motor and the associated circuitry so as to be able to handle such excessive current spikes including those occurring during the full rotational speed reversal. So, for instance, there have been employed in the electric motor design permanent magnets that are less affected by current surges, larger motor casings that are able to dissipate heat resulting from associated power losses because of their larger surface area, and control transistors/module assemblies capable of dissipating and resisting power surges. However, such overdesign of the electric motor and the associated circuitry and components thereof results in an increased size, weight and cost of the electric motor assembly.
In the alternative, it has been proposed to rate-limit the motor command to prevent large electric current spikes. However, if the rate limiting is sufficiently high to prevent large current spikes for large motor command changes, the electric motor response to small motor command changes suffers as well.
There are also already known, for instance from the U.S. Pat. Nos. 4,217,528, 4,494,181, 4,527,103 and 4,544,869, various control arrangements that operate the control switches of the H-bridge motor control circuit in response to external command signals. However, these known control arrangements suffer of one or more of the above-discussed disadvantages, that is, either they do not provide for an automatic control of the electric current flowing through the winding at all, or the electric current control is applied across the board, that is, not only for large motor command reversals but also for small ones.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to develop an arrangement for controlling the operation of an H-bridge reversible D.C. motor, which arrangement does not possess the disadvantages of the known arrangements of this type.
Still another object of the present invention is so to construct the arrangement of the type here under consideration as to be able to automatically recognize reversal commands which would result in excessive current spikes and to modify the control of the motor in response to such recognition in such a manner as to avoid such excessive current spikes.
It is yet another object of the present invention to design the above arrangement in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and reliable in operation nevertheless.
A concomitant object of the present invention is to provide a method of controlling the operation of the H-bridge reversible D.C. motor that renders it possible to achieve an automatic limitation of the current flowing through the electric motor winding during sense or rotation reversals in the event that such current flow would be excessive.